Well, maya has some variables connected to the internalVar command, but don't use that.
What you want is the getenv command to quey environment variables defined in your OS.
So for example "getenv USERNAME;" without the quotes, will return "C:/Users/ADMIN"
Then concatenate the folder and/or file you want. In your case the following line with quote and all:
(getenv USERNAME) + "/Desktop/Test.ma"
Or did I misunderstand you, are you in fact asking how to retrieve the path to the file that is open?
that would be "file -query -location;"